Cross the bridge

You went out for an internship somewhere in the mountains (Weird). By the time you started working there, you got to know 3 people, the professor, his lady assistant and an old man.One fine day you pulled a lever which had a danger sign on it.
That button opened the chamber which was heavily crowded with zombies.The only way to escape is to cross the bridge and cut the ropes. You need to take all the people along with you to the other side of the bridge safe and alive ! !But there is a problem, everyone can’t cross the bridge with same speed. Everyone will take different time to cross. You take 1 minutes, the professor takes 10 minutes, her lady assistant takes 2 minutes and the old man takes 5 minutes to cross the bridge.According to the professor’s calculation, zombies will reach upto them within 17 minutes.But there are certain difficulties to cross the bridge. Only 2 people can cross the bridge at once otherwise the bridge will break.Also since its night, so you need a lamp to walk through the dark bridge. You only have one lamp.So try to solve the riddle by making everyone cross the bridge safe ! !

1. First, you and lady assistant go across which will take 2 minutes.
Total Time Taken: 2 mins

2.You come back with the latern. It will take 1 minutes.
Total Time Taken: 3mins

3.You send professor and old man with latern across the bridge. It will take 10 minutes
Total Time Taken: 13 mins

4. The lady assistant who is already across the bridge will come back with latern. It will take 2 minutes.
Total Time Taken: 15 mins

5. You and lady assistant go across the bridge. This will take 2 minutes.
Total Time Taken: 17mins

Now as all of you are across the bridge, quickly cut off the bridge ropes. Congratulations! You're safe